The present invention relates to compounds having antiviral activity, processes for their preparation and pharmaceutical compositions containing them.
The compound 9-(4-hydroxy-3-hydroxymethylbut-1-yl)guanine of formula (A) 
is disclosed in Synthetic Communications, 2(6), 345-351 (1972) but no pharmaceutical activity has been indicated for the compound in this or any other published document. We have repeated the synthesis of the compound as described in the above publication, and have shown that the product is a mixture of the compound of formula (A), its monobenzyl ether and its dibenzyl ether, this mixture having a melting point and uv spectrum in agreement with those reported in the publication for the supposedly xe2x80x98purexe2x80x99 compound of formula (A). Our analysis of the product produced by the above synthesis showed that it contained 45-50% by weight of the compound of formula (A), 45-50% by weight of the monobenzyl ether and 5% or less by weight of the dibenzyl ether.
By different synthetic routes, we have prepared the compound of formula (A) in a substantially pure form and have found that it has anti-viral activity. This activity is also shown by certain derivatives of the compound of formula (A).
According to the present invention there is provided a compound of formula (I) 
or a salt, phosphate ester or acyl derivative thereof, in which X represents chlorine, straight or branched chain C1-6 alkoxy, preferably methoxy, phenoxy, phenyl C1-6 alkoxy, xe2x80x94NH2, xe2x80x94OH or xe2x80x94SH with the proviso that, when X is xe2x80x94OH, the compound of formula (I) is in a purity state of greater than 50% by weight, of pure compound.
The term xe2x80x98acyl derivativexe2x80x99 is used herein to include any derivative of the compounds of formula (I) in which one or more acyl groups are present. Such derivatives include biological precursors of the compounds of formula (I) in addition to those derivatives which are per se biologically active.
Examples of acyl derivatives of the compounds of formula (I) are those wherein one or both of the hydrogens in the acyclic OH groups, and/or one of the hydrogen atoms in the xe2x80x94NH2 group, are replaced by 
wherein R is hydrogen or an alkyl, aryl, aralkyl or heterocyclyl group.
Examples of alkyl groups R include straight and branched chain groups containing up to 18 carbon atoms, preferably up to 6 carbon atoms. Particular examples are methyl, ethyl, t-butyl and pentyl.
Examples of aryl groups R include phenyl optionally substituted with up to five preferably up to three groups.
Examples of aralkyl groups R include phenyl-C1-6 alkyl groups such as benzyl.
Examples of heterocyclyl groups R include single or fused rings containing one or two hetero-atoms in each ring, selected from oxygen, nitrogen and sulphur.
Examples of phosphate esters of the compounds of formula (I) include those where one or both of the acyclic xe2x80x94OH groups are replaced by 
or salts thereof, or where the two xe2x80x94OH groups are replaced by a bridging 
group
Salts, phosphate esters and acyl derivatives of the compounds of formula (I) are preferably pharmaceutically acceptable, but non-pharmaceutically acceptable compounds are also within the scope of the present invention, since these are useful as intermediates in the preparation of pharmaceutically acceptable compounds.
The compounds of formula (I) are defined herein as including tautomers of formula (I), wherein the xe2x80x94OH and xe2x80x94SH substituents are replaced by xe2x95x90O and xe2x95x90S substituents respectively.
A particular group of compounds of the invention are those of formula (II) 
or pharmaceutically acceptable salts thereof, in which X is as defined in formula (I), and each of R1, R2 and R3 represents hydrogen or an acyl group of formula 
in which R4 is C1-18 alkyl or imidazolyl, or R1 or R2 represents a phosphate ester group of formula 
or R1 and R2 together represent a 
bridging group.
Subject to the aforementioned purity proviso in relation to compounds of the invention, a preferred compound of the present invention is the compound of formula (A) 
or a salt or acyl derivative therof.
In a further aspect of the invention there is provided a compound of formula (A) in a purity state of greater than 60% preferably greater than 80% more preferably greater than 90% and particularly preferably more than 95% by weight of pure compound.
In yet a further aspect of the invention, there is provided an isolated, substantially completely pure compound of formula (A), or a pharmaceutically acceptable salt thereof.
The invention also provides a compound of formula (A) in crystalline form having a melting point of 275-277xc2x0 C.
The compounds of the present invention have antiviral activity, and are potentially useful in the treatment of infections caused by herpes viruses, such as herpes simplex type 1, herpes simplex type 2 and varicella zoster viruses.
Accordingly, the present invention also provides a compound of formula (I) or a pharmaceutically acceptable salt, phosphate ester or acyl derivative thereof, for use as an active therapeutic substance, and in particular for use in the treatment of viral infections. In this aspect of the invention, the compounds of formula (I) are not subject to the aforementioned purity proviso.
Examples of pharmaceutically acceptable salts of the compounds of formula (I) are those formed with organic bases, preferably with amines such as ethanolamines or diamines; and alkali metals, such as sodium and potassium; and acid addition salts formed with a pharmaceutically acceptable acid such as hydrochloric acid, orthophosphoric acid and sulphuric acid.
The compound of formula (A) or a salt thereof may be prepared by converting the group X in a compound of formula (III). 
in which X, excluding xe2x80x94OH, is as defined in formula (I); Ra and Rb, which may be the same or different, are each hydrogen or Oxe2x80x94 protecting groups, preferably acyl groups; and Y is chlorine or xe2x80x94NHRc, in which Rc is hydrogen or acyl, to an xe2x80x94OH group by means of hydrolysis, preferably acid hydrolysis, when X is other than NH2, or, when X is xe2x80x94NH2, by means of a deaminase reaction, or when Y is chlorine and X is xe2x80x94OH, converting Y to a xe2x80x94NH2 group by reaction with ammonia under pressure in accordance with known methods, and subsequently, if desired, converting the compound of formula (A) to a salt thereof by treatment with an acid or base.
Acyl groups Ra, Rb and Rc may be those of formula 
as hereinbefore defined.
Examples of groups Ra and Rb in formula (III) are acetyl and cyclic acetal such as isopropylidene, is preferably acetyl or hydrogen.
A preferred process for preparing the compound of formula (A) comprises treating a compound of formula (III) in which X is chlorine, Y is xe2x80x94NH2 and Ra and Rb are each acetyl, with aqueous mineral acid, preferably hydrochloric acid.
Compounds of formulae (III) when each of Ra, Rb and Rc is hydrogen or acyl, are themselves compounds of the invention, having the additional utility as intermediates for the preparation of the compound of formula (A).
In a further aspect of the invention, compounds of formula (I) or acyl derivatives thereof, together with, compounds of formula (III), may be prepared by treating a compound of formula (IV). 
in which X is as defined in formula (I) and Y is as defined in formula (III), with a compound of formula (V) 
in which Ra and Rb are as defined in formula (III) and Z is a leaving group such as Cl, Br, or I, preferably Br.
Compounds of formula (IV) are either known compounds or can be made from known compounds by known methods.
Compounds of formula (V) in which Z is bromine may be prepared by brominating a compound of formula (VI). 
preferably by treatment with carbon tetrabromide and triphenylphosphine in an organic, aprotic solvent, such as dimethylformamide.
Compounds of formula (V) in which Z is Cl or I may be prepared in an analogous manner.
Compounds of formula (VI) in which Ra and Rb are identical may be prepared according to the following schematic process. 
Acyl derivatives of compounds of formula (I) may also be prepared by acylating an optionally protected compound of formula (I) in accordance with conventional acylating processes known in the art, and where necessary, deprotecting the resulting product.
The acylation reaction may be carried out by using an acylating agent containing a 
wherein R is as hereinbefore defined.
In a particular aspect of this process, the acylating agent contains the 
in which R4 is C1-18 alkyl, or is N,Nxe2x80x2-carbonyldiimidazole.
Examples of acylating agents suitable for the above process are carboxylic acids, acid halides or acid anhydrides, preferably anhydrides or acids.
When the acylating agent is a carboxylic acid, a coupling agent such as dicyclohexylcarbodiimide should be included, but this is not necessary when the acylating agent is an acid anhydride.
The acylation reaction may produce a single acyl derivative of a compound of formula (I), or a mixture of derivatives, depending on a number of factors, such as the relative amounts and chemical natures of reactants, the physical conditions of the reaction, and the solvent system. Any mixture produced in this way may be separated into its pure components using standard chromatographic techniques.
The above described acylation process of the invention can yield mono-, di-, or tri-acylated derivatives of compounds of formula (I) according to the form of protection/deprotection utilised. The following are examples of products obtained by different methods:
(a) Di-acylated derivatives of the two acyclicxe2x80x94OH groups may be obtained by direct acylation of unprotected compounds of formula (I) or acylation of protected intermediates of compounds of formula (I) in which the xe2x80x94NH2 group is protected by, for example, a monomethoxytrityl group, and subsequent deprotection by treatment with acid.
(b) Mono-acylated derivatives of one of the acyclic xe2x80x94OH groups may be obtained by acylation of protected intermediates of compounds of formula (I) in which the xe2x80x94NH2 group and the other acyclic xe2x80x94OH group are both protected by, for example, monomethoxytrityl groups, and subsequent deprotection by acid treatment.
(c) Mono-acylated derivatives of the NH2 group may be obtained by acylation of protected intermediates of compounds of formula (I) in which both acyclic xe2x80x94OH groups are protected by, for example trimethylsilyl groups, and subsequent deprotection.
The various protected intermediates of compounds of formula (I) may be prepared in accordance with standard procedures by, for example, treatment of the compounds with monomethoxytrityl chloride (for processes (a) and (b)) or with chlorotrimethylsilane (for process (c)).
Protected intermediates of compounds of formula (I) may also be used to prepare phosphate esters of the compounds.
Accordingly, in a further process aspect of the invention, there is provided a process for preparing a mono-phosphate ester of a compound of formula (I) which comprises treating a protected intermediate of the compound of formula (I) in which one of the acyclic xe2x80x94OH groups and the xe2x80x94NH2 group are protected, preferably by monomethoxytrityl groups, with cyano ethyl phosphoric acid and subsequently deprotecting the resultant product by treatment with acid, preferably acetic acid.
If desired, the reaction product after treatment with cyano ethyl phosphoric acid is treated with aqueous ammonia, which yields the ammonium salt of the phosphate ester as the final product.
Compounds of formula (I) or salts thereof may also be prepared by hydrolysing the 1,3-dioxane ring of a compound of formula (VII). 
in which X is as defined in formula (I) and Rc is as defined in formula (III), provided that Rc is not acyl when X is other than OH, and subsequently, if desired, converting the compound of formula (I) thus formed to a salt by treatment with an acid or base.
When Rc is an acyl group, a basic N-deprotection step is required to form the compound of formula (A). This can be carried out prior to or after hydrolysis by treatment with, for example, (i) a solution of NaOMe in CH3OH or (ii) a solution of NH3 in CH3OH.
Preferably the hydrolysis of compounds of formula (VII) is carried out in acid medium. The compounds of formula (VII) in which X is alkoxy, phenoxy, phenylalkoxy or xe2x80x94SH are conveniently prepared in situ by reacting the compound of formula (VII) in which X is chlorine with an additional reactant containing an X1 substituent, wherein X1 is alkoxy, phenoxy, phenylalkoxy or sulphur. These intermediates can then be hydrolysed to compounds of formula (I) without isolating them from the reaction mixture.
The additional reactant containing the X1 moiety may be a sodium alkoxide, phenoxide or phenylalkoxide, or sodium hydrosulphide (when X1 is sulphur).
Acid hydrolysis of a compound of formula (VII) in which X is chlorine willyield a compound of formula (I) in which X is chlorine, or a compound of formula (A) depending on acid strength and reaction conditions.
For example, treatment of the compound of formula (VII) in which X is chlorine with dilute HCl (1.0M) at 60xc2x0 C. for 24 hours or with 2 M HCl under reflux for 1.5 hours, will yield the compound of formula (A). Treatment of the same compound of formula (VII) with 2 M HCl in tetrahydrofuran at room temperature will yield the compound of formula (I) in which X is chlorine.
If desired, the compound of formula (VII) in which X is chlorine may be converted to the compound of formula (VII) in which X is amino, prior to acid hydrolysis.
The conversion may be achieved by treatment with sodium azide in dimethylformamide to form an azido intermediate in which X is replaced by an azide moiety, followed by reduction of the intermediate with ammonium formate/palladium-on-charcoal in methanol.
The intermediate compound of formula (VII) in which X is chlorine and Rc is hydrogen may be prepared by treating a compound of formula (VIII). 
with a compound of formula (IX) 
The reaction may be carried out in an inert organic solvent, preferably dimethylformamide, in the presence of an inorganic base, preferably potassium carbonate.
The compound of formula (VIII) may itself be prepared by brominating a compound of formula (X) 
The reaction is preferably carried out by treating the compound of formula (X) with carbon tetrabromide and triphenylphosphine in an organic, aprotic solvent such as dimethylformamide.
The compound of formula (X) may itself be prepared by treating a compound of formula (XI) 
with 2,2-dimethoxypropane and p-toluenesulphonic acid in the presence of acetone or tetrahydrofuran.
The compounds of formulae (IX) and (XI) are known compounds or can be prepared from known compounds by known methods.
The compounds of formulae (VII), (VIII) and (X) are novel intermediates and as such form further aspects of the present invention.
A compound of formula (I) or pharmaceutically acceptable salt, phosphate ester or acyl derivative thereof may be formulated for use in a pharmaceutical composition. Accordingly, in a further aspect of the invention, there is provided a pharmaceutical composition which comprises a compound of formula (I) or pharmaceutically acceptable salt, phosphate ester or acyl derivative thereof together with a pharmaceutically acceptable carrier or excipient.
A composition which may be administered by the oral route to humans may be compounded in the form of a syrup, tablet or capsule. When the composition is in the form of a tablet, any pharmaceutical carrier suitable for formulating such solid compositions may be used, for example magnesium stearate, starch, lactose, glucose, rice, flour and chalk. The composition may also be in the form of an ingestible capsule, for example of gelatin, to contain the compound, or in the form of a syrup, a solution or a suspension. Suitable liquid pharmaceutical carriers include ethyl alcohol, glycerine, saline and water to which flavouring or colouring agents may be added to form syrups. The compounds may also be presented with a sterile liquid carrier for injection.
The composition may also be formulated for topical application to the skin or eyes.
For topical application to the skin, the composition may be in the form of a cream, lotion or ointment. These formulations may be conventional formulations well known in the art, for example, as described in standard books of pharmaceutics and cosmetics, such as Harry""s Cosmeticology published by Leonard Hill Books and the British Pharmacopaeia.
The composition for application to the eyes may be a conventional eye-drop composition well known in the art, or an ointment composition.
Preferably, the composition of this invention is in unit dosage form or in some other form that the patient may administer to himself a single dose. A suitable dosage unit might contain from 50 mg to 1 g of active ingredient, for example 100 to 500 mg. Such doses may administered 1 to 4 times a day or more usually 2 or 3 times a day. The effective dose of compound will in general be in the range of from 1.0 to 20 mg/kg of body weight per day or more usually 2.0 to 10 mg/kg per day.
In a further aspect of the invention there is provided a method of treating viral infections in a human or non-human animal, which comprises administering to the animal an effective, non-toxic amount of a compound of formula (I) or a pharmaceutically acceptable salt, phosphate ester or acyl derivative thereof.